This invention relates to ultrasonic welding methods and apparatus and, more particularly, concerns an ultrasonic welding method and apparatus for thermoplastic film and fabric material which is welded on a continuous basis by being fed through a weld station formed by a resonant horn and an oppositely disposed anvil, usually a cylindrical roller. As a result of the dissipation of ultrasonic energy coupled by the horn to the material, welding of the material occurs. For instance, two or more sheets of thermoplastic material can be joined or seamed together. Ultrasonic apparatus of this type are well established, see for instance, U.S. Pat. No. 3,733,238 issued to D. D. Long et al., May 15, 1973, "Apparatus for Vibratory Welding of Sheet Material".
The present invention, quite specifically, concerns a closed-loop continuous feed welding method and apparatus in which the ultrasonic energy density coupled to the material is preselected and is maintained constant at that level despite variations in feed speed, weld force or variations and inconsistencies of the material. The closed-loop system, as will be shown hereafter, includes means for developing a feed-back signal which is applied to a feedback controlled electrical power supply, or to a force means controlling the engagement force between the horn and the material, or to both, for maintaining the ultrasonic energy density at a preset level by sensing the feed speed and the power provided to the resonant horn, and processing in a control circuit, which receives a preset ultrasonic energy density signal, signals commensurate with both these operating parameters.
Controlling in an ultrasonic apparatus the ultrasonic power coupled to a workpiece as a function of feed speed is not entirely new as seen, for instance, in U.S. Pat. No. 3,666,599 issued to E. G. Obeda, dated May 30, 1972, entitled "Sonic or Ultra-sonic Seaming Apparatus". This patent discloses an ultrasonic seaming or welding apparatus in which the power provided from the ultrasonic power supply is varied as a function of material feed speed by using for power control a rheostat which is coupled to a foot operated speed control. However, this patent represents an open-loop system in which the speed versus ultrasonic power from the power supply is empirically established and is not altered for different materials, for changes in engagement force, or for variations present in a roll of material. These short-comings are overcome by the closed-loop welding method and apparatus disclosed herein.